1. Field of the Invention
The role of catalytic cracking in fluidized and moving bed systems is well known at this stage of the art, having undergone progressive development since early 1940. Until recent years catalytic cracking operations have been forced to use a silica-alumina cracking catalyst which, by today's standards, is considerably less active and particularly is considered less selective for performing the catalytic cracking of the hydrocarbon charge to produce gasoline product. Thus, considerable difficulty has been encountered in the prior systems in obtaining high yields of conversion products without excess production of the carbonaceous contaminants.
The present trend in catalytic cracking operations is concerned with those systems which will use more active and selective cracking catalysts, such as those comprising crystalline zeolites, for performing the conversion of one or more high boiling hydrocarbon fractions of the same or different boiling range and coke producing characteristics to gasoline boiling range products. Thus, crystalline zeolite cracking technology necessarily requires using much more sophisticated cracking systems than those known or disclosed in the prior art in order to take full advantage of the catalyst's conversion capabilities. Many prior art systems and those converted for the use of high activity crystalline zeolite cracking catalysts have produced an inefficient operation, causing undue catalyst regeneration, excessive recycle of unconverted charge and general inefficient use of the catalyst composition.
The invention defined herein is concerned with an improved apparatus and sequence of conversion steps which will more efficiently utilize the capabilities of a crystalline zeolite cracking catalyst of high activity and high selectivity.
2. Description of the Prior Art
Fluid catalyzed cracking systems and zeolite cracking catalysts are well known in the art and are disclosed in many U.S. patents, including U.S. Pat. Nos. 3,748,251; 3,791,962; 3,849,291; 3,856,659; 3,894,933; 3,894,943; 3,894,935; 3,907,663; and 3,926,778, all of which are incorporated by reference.